It is known to detect motion between fields of identical parity in a two-to-one interlaced television standard, as between successive even fields, or between successive odd fields. In detecting images in motion, information from a first frame is subtracted from information from a second frame which is spatially aligned with the information from a first frame. When the result of this subtraction is zero, no motion is deemed to be present. When there is a difference, motion is deemed to be present. The present inventor and others have developed a frame-by-frame based motion detector which is disclosed in commonly assigned U.S. Pat. No. 4,967,271 in conjunction with FIGS. 3, 4 and 5 thereof, the disclosure of which being hereby incorporated by reference.
In the case of a two-to-one interlaced television signal standard, such as NTSC or PAL, the foregoing motion detection process does not work on a field-by-field basis because, even in the case of static picture content, the interlace process provides different information between even and odd field scan lines. For example, a conventional television frame at e.g. a 30 Hz repetition rate in the case of the NTSC standard is composed of two 60 Hz fields, e.g. F0 and F1. Each field includes 262.5 scan lines. Each scanning line of an odd or even field is separated by an unilluminated strip or band. Successive fields are offset by one-half line, so that the scan lines of the next field (e.g. even field) occupy the unilluminated strips of the present field (e.g. odd field). This interlaced picture standard has been followed to minimize perception of 30 Hz flicker in the resultant picture display.
If spatially aligned information between adjacent odd and even fields are subtracted, a difference will result for every instance of a vertical transition as well as for a motion condition. Consequently, motion detection systems following the prior art approach do not work on a field by field basis within a two-to-one interlaced standard.
While the prior motion detection methods and approaches work for some television signal processing applications, there are a number of emerging applications and uses which require detection of motion within the interlaced frame, rather than on a frame-by-frame basis. One example is in the case of multi-dimensional signal processing of video such as line doubling wherein information from adjacent fields is being interpolated into new information for the doubled line picture. Another example is the conversion of one television picture signal in a first standards format such as NTSC into a signal following a second standards format having a different line scan rate such as PAL. A further emerging application is in the field of digital processing, special effects, and like applications requiring motion compensation.
One other important example is within a process for detecting whether a video source originated with film or with a television camera viewing a live picture source, for example. One example of such a process is disclosed in commonly assigned U.S. Pat. No. 4,982,280 entitled "Motion Sequence Pattern Detector for Video", the disclosure of which being hereby incorporated by reference. In this prior approach a sequence of picture images is analyzed to determine the presence of three-to-two pull down ratio which is characteristic of a film-to-video source of the video information. An exemplary sequence is illustrated in FIG. 1 which shows the repetition of film frames A, B, C, D, E, F, etc., on a three-to-two pull down ratio basis. By detecting the three-to-two pull down sequence as shown in FIG. 1, line doubling based on a field by field basis became entirely practical.
A hitherto unsolved need has arisen to detect film sequences in other standards, such as a 625 line, 50 Hz format standards (e.g. PAL). In the
format the field rate is 50 fields per second, and a film sequence (originally filmed at 24 frames per second) is speeded up for playback at 25 frames per second. For this reason, there is no pull down ratio in the
format, and for this reason one cannot detect film sequences in PAL with frame motion circuitry. Also, in the PAL format each film frame occupies an odd field and an even field, with the next odd and even fields carrying the next film frame. Thus, a hitherto unsolved need has arisen for a motion detection method and apparatus which is capable of detecting motion between successive fields of opposite polarity.
Also, the need to detect motion on a field by field basis has arisen in the NTSC standards format when film has been transferred to video format and then subjected to electronic editing. In those instances of film to video transfer and subsequent video edit, the three-to-two pull down ratio is frequently disturbed by an editing splice between fields which are inconsistent with the film pull down ratio. This situation is illustrated in FIG. 2 wherein the three-to-two pull down arrangement of film frames to video fields is shown to have an edit point and resultant pull down ratio discontinuity between film frames B and P. The line doubler described in the referenced prior U.S. Pat. No. 4,982,280 could remain in film mode for e.g. up to five fields, with the undesirable result of creating an objectionable line pattern in the resultant video display immediately following the edit point. In order to correct this situation, it becomes important to detect differences of information between even and odd fields.
Also, extremely complex systems and methods are being proposed to preserve the integrity of a video image when standards conversion is performed on film-originated subject matter. One approach known in the prior art is to provide e.g. 24 frame memories which is very expensive. There is something unnecessarily complicated in processes which begin at an original image rate of 24 images per second, which become processed at a field rate of 60 (or 50) fields per second, and then revert to 24 (or 25) images per second. It is therefore important to know if the original subject matter was film and to detect that fact; and further, when there is a discontinuity attributable to electronic editing, to detect that fact and immediately compensate for it. This need exists whether the conversion is from a 60 Hz to 50 Hz standard, or vice-versa.
The need to detect motion on a field-by-field basis also arises in the context of systems which change the time scale. Many contemporary video recorder/playback units provide an option to play back the images at an accelerated rate of 1.5 to 2 times the original speed. In these processes the resultant picture images may be improved upon by detection of motion on a field-by-field basis.
Another need for detection of motion between adjacent interlaced fields has arisen in conjunction with digital encoding and decoding of video information for data compression, particularly in the field of high definition television (HDTV). In such applications, it is useful to modify the coefficient algorithm coefficients whenever film sequences are present. Detection of film sequences, and irregular edit points, has thus arisen as a hitherto unsolved need.
Thus, from the foregoing the reader will appreciate that a hitherto unsolved need has arisen for a method and system to detect motion on a field-by-field basis from an interlaced video source (irrespective of the fact that the signal may be progressively displayed or transmitted).